Some hotspots (e.g., Hawaii, Iceland) have been active for millions to tens of millions of years and are thought to be the surface manifestation of partial melting in a mantle plume. To understand the evolution of hotspot magmatisms it is necessary to determine how the compositions and productivity of magmatism vary with time. In this study, temporal geochemical variation in the Tertiary Icelandic magmatism is elaborated based on comprehensive analytical dataset. We revealed temporal changes in composition of magma sources and identified three distinct end-member components in this magmatism: one is the upper mantle peridotite and the other two are crustal lithologies. We also found the correlation between contribution from each end-member component and rate of magma production: higher magma productivity is coincident in time with larger contribution from recycled-crustal lithologies. The crustal lithologies have melting points lower than that of peridotite and should result in higher melt productivity at a given temperature in the melting region than melting of source dominated by peridotite. We therefore conclude that correspondence between productivity and the compositions of the Tertiary Icelandic lavas could be due to the periodic entrainment of recycled crustal lithologies into the pulses of Iceland mantle plume at its source region.
The two parallel loci of recent Hawaiian volcanoes, Kea and Loa, have been regarded as the best targets to interpret the chemical structure of an upwelling mantle plume derived from the lower mantle. Here we show that the Sr-Nd-Hf-Pb isotopic data of the shield building lavas along the Loa locus form a systematic trend from the main shield stage of Koolau (>2.9 Ma) to the active Loihi volcanoes. The abrupt appearance of Loa type magmatism should be attributed to the transient incorporation of the relatively dense recycled material and surrounding less degassed lower mantle material that accumulated near the core-mantle boundary into the upwelling plume. This episodic involvement could have been trigged by episodic thermal pulses and buoyancy increases in the plume. The continuous appearance of Kea-type lavas during the long history of Hawaiian-chain magmatism and the larger magma volume of Kea-type lavas relative to that of the Loa-type lavas in the last 3 Myr indicate that the Kea locus is closer to the thermal centre of the Hawaiian plume relative to that of the Loa locus. (24, Oct., 2007)